In general, a gear, a shaft, a pulley, a constant velocity joint, and the like which are used for various gear transmission devices such as an automobile transmission and a differential as well as structural parts such as a crankshaft and a connecting rod are formed into an ultimate shape after being subjected to a processing such as forging and a cutting processing in this order. Since a cost required for the cutting processing has a large proportion in a manufacture cost, a steel material forming the structural parts is required to have excellent machinability.
In the structural parts as described above, predetermined strength is ensured by forming the structural parts into the ultimate shape, and then subjecting them to a surface hardening treatment such as carburizing and carbonitriding (including atmospheric, low pressure, vacuum, and plasma carbonitriding) and, according to the necessity, followed by quenching-tempering, induction heating and quenching, or the like. However, a lowering in strength can occur during such processings. Particularly, there is a problem that the steel material is liable to be lowered in strength in a direction perpendicular to a rolling direction (this direction is generally referred to as “transverse direction”).
Lead (Pb) has heretofore been known as an element improving the machinability without reducing the strength of steels for machine and structural use, and Pb is remarkably effective for the machinability improvement. However, since Pb is harmful to living body and has problems in processings such as Pb fume during melting and cutting waste, there is a demand for excellent machinability which is achieved without adding Pb (Pb free).
As a technology for ensuring excellent machinability without adding Pb, a steel material having an S content increased to about 0.06% has been known. However, in this case, since there is a problem that mechanical characteristics (toughness, fatigue strength) are liable to be lowered, the increase in S content has limitation. Such problem is considered to occur due to a reduction in toughness in transverse direction that is caused by expansion of sulfide (MnS) in a rolling direction. Particularly, in the parts which are required to have high strength, it is necessary to reduce the S content to an amount as small as possible. In view of above, it is necessary to establish a technology for improving machinability without actively adding Pb and S. Under the circumstances, various technologies for achieving excellent machinability without actively adding Pb and S have been proposed.
Incidentally, in a production process of a gear which is one of the structural parts, a steel (material) for machine and structural use generally undergoes forging, rough cutting by hobbing, and finishing by shaving, followed by a heat treatment such as carburizing and polishing (honing) again. However, since heat treatment distortion frequently occurs in such process, it is difficult to adjust such heat treatment distortion only by the polishing, and dimensional accuracy of the part is reduced in some cases. Recently, excellent dimensional accuracy has been in demand in view of countermeasure for suppressing noise of gears in use. As the countermeasure for this, grinding (hard finishing) is in some cases performed in advance of the polishing.
In any of the production processes, the considerably large number of process steps is required, and a cost required for the cutting and grinding is increased, thereby raising a great need for cost reduction of the process as a whole. Therefore, a cost reduction is required in each of the process steps, and there is a great expectation for a steel that enables such cost reductions. Particularly, in view of the high tool cost in the hobbing that is performed in both of the processes, there is a great expectation for a technology for improving tool life.
The hobbing is equivalent to intermittent cutting, and, as a tool for the hobbing, a tool obtained by coating AlTiN or the like on a high speed tool steel (hereinafter sometimes referred to as high speed tool) is mainly used at present. In contrast, since a tool obtained by coating AlTiN or the like on a cemented carbide (hereinafter sometimes referred to as carbide tool) has a problem that edge chipping often occurs when used for normalized materials, the tool is usually used for continuous cutting such as turning.
Since the intermittent cutting and the continuous cutting are different from each other in cutting mechanism, the tool suitable for each of the cuttings is selected, and it is desirable that the steel for machine and structural use used as a material to be machined has a property for exhibiting excellent machinability in both of the cuttings. However, gear cutting by the hobbing (intermittent cutting) using the high speed tool has a drawback that the tool is subject to oxidation and wear at a low speed and a low temperature as compared to the case of turning which is the continuous cutting using the carbide tool. Therefore, particularly a tool life among the machinability is required to be improved in the steel for machine and structural use undergoing the intermittent cutting such as the hobbing.
However, actual situation is such that an improvement in machinability in the intermittent cutting, particularly, a technology for improving machinability in the intermittent cutting in the case of low cutting speed, has not been established yet. As a technology for improving machinability, for example, JP-A-2001-342539 discloses a steel material excellent in intermittent cutting (tool life) at a high speed (cutting speed: 200 m/min or more) which is obtained by including 0.04 to 0.20% of Al and 0.0030% or less of O. An intermittent high speed cutting steel which achieves excellent intermittent cutting at high speed was realize by the technology. However, the technology is basically on the assumption of cutting with a carbide tool (with the use of carbide tool P10 (JIS B4053)), and machinability with a high speed tool at a low speed cutting (low temperature cutting) thereof is insufficient.
Also, JP-A-2003-226932 discloses a steel material that enables excellent high speed cutting in turning (continuous cutting) and milling (intermittent cutting) by containing 0.001% to 0.040% of S, 0.04% to 0.20% of Al, and 0.0080% to 0.0250% of N as well as by controlling a ratio ([Al]/[N]) between an Al content [Al] and a N content [N] to 2.0 to 15.0. However, the technology is basically on the assumption of cutting with a carbide tool (with the use of carbide tool P10) as is the case with the above-mentioned technology, and machinability with a high speed tool at a low speed cutting thereof is insufficient.
JP-A-11-229032 discloses an improvement in machinability such as a capability of drilling that is achieved by controlling a chemical composition in a steel for soft nitriding to high Cr (0.5 to 2%) and high Al (0.01 to 0.3%) as well as by controlling a maximum diameter of Ti carbosulfide in the steel to 10 μm or less. However, the publication does not disclose any description about the intermittent cutting with high speed tool at low speed.